There's a lot of silliness in the last few days regarding NASA's plan to crash a probe into the moon to kick up water crystals that we can observe from earth. In the 1960s, the US and USSR crashed probes into the moon. These were known as "hard landings." But now, thanks to the incredible power of stupid, LCROSS's hard landing is referred to as "bombing the moon" (as opposed to tagging the moon) and the chatter is even sillier than that surrounding the switching on of the Large Hadron Collider.

The committee reviewing NASA's future plans has put out a preliminary report and it looks like the "flexible path" proposal that involves human missions to asteroids and Mars orbit is the curret front-runner.

But the final report will address other stuff like safety not covered in this preliminary report.

Asteroid missions could lead to mining asteroids for water and other materials which could make a huge difference to the cost of space travel.

not sure if this is the right place to ask, but i'm looking for specific info. suppose an astronaut rather forcefully gets knocked off the ISS during a spacewalk, in the direction of earth, will the astronaut get caught in earth's gravity & begin to decelerate? and how long would it take before they start to burn up?i've scoured the 'nets to find the answer, but it seems my google-fu is weak...

suppose an astronaut rather forcefully gets knocked off the ISS during a spacewalk, in the direction of earth, will the astronaut get caught in earth's gravity & begin to decelerate?

The astronaut and the ISS are both already (and remain) "caught in earth's gravity", to the extent that they're in orbit.

Their orbital velocity is about 27,500 km/h (i.e. 7.7 km/s).

Even being "rather forcefully" knocked off wouldn't make much difference to the astronaut's personal orbit: they might be travelling at 10 km/hour away from the space station, but they're still travelling at 27,500 km/h around the earth: which would be enough to remain in orbit.

Their additional velocity would make their orbit slight eccentric (more elliptic and less circular). They wouldn't crash unless their orbit became so very elliptic that it intersected the atmosphere (however 10 km/hour is negligible compared to 27,500 km/h, so it's hardly elliptic at all).

Because their new orbit is elliptic, they'd start by drifting away from the ISS but then might even start to drift towards it again, because an elipse is sometimes inside and sometimes outside the corresponding circle (or at least, this factoid was suggested in the recent SF novel _Anathem_).

and how long would it take before they start to burn up?

Google for "stationkeeping", "orbit decay", "low earth orbit".

There are some details at http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/SatelliteOrbitalDecayCalculations.pdf but I can't be bothered to crunch the numbers. Deceleration is proportional to atmospheric drag which is proportional to the object's surface area; if an astronaut's mass-to-surface-area ratio is less than that of the ISS, then their orbit will decay more quickly than the ISS's.

The ISS orbits at 350 km above Earth's surface, orbiting the Earth once every 90 minutes. The surface area of the space station is 1 acre (the solar arrays), i.e. 4,000 sq meters, and it weighs 300 tons, so its mass-to-area ratio is about 700 kg per square metre. The surface area of an astronaut is maybe 2 square metres, and their weight about 200 kg, so their mass-to-area ratio is about 100 kg per square metre. I'd therefore guess that the deceleration due to atmospheric drag would be 10 times greater for an astronaut than for the ISS.

Figures 3 and 4 of SatelliteOrbitalDecayCalculations.pdf suggest that at 350 km, the lifetime of an object whose mass-to-area ratio is 100 kg/m^2 is about 80 days.

To answer your question then: if an astronaut is orbiting near the ISS, then I'd expect that it might take their orbit about 80 days to decay due to atmospheric drag.

I'm uncertain how fast they'd need to fly away from the space station, to get an orbit that's eccentric enough for that to have a significant effect, but my guess is as follows: that they'd need an orbital eccentricity of only about 170 km for that to have a major effect (give that atmospheric drag is very significant at 180 km above the earth); and that given an orbital period of 90 minutes, I think that acquiring an eccentricity of 170 km may require a delta-v on the order of a few hundred kilometres/hour. So, they would need to be more than just "rather forcefully gets knocked": either that knock/strike would kill them, or, their acceleration to get a delta-v of a few hundred kilometres/hour would need to be constant/sustained for at least several seconds instead of being instantaneous.

SAFER weighs approximately 83 lb (38 kg) and can provide a total delta-v of at least 10 ft/s (3 m/s): http://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue

Other Crew Self Rescue (CSR) devices of which prototypes have been developed include an inflatable pole, a telescoping pole, a bi-stem pole and a bola-type lasso device (astrorope) the drifting astronaut could throw to hook to the space station: http://en.wikipedia.org/wiki/Manned_Maneuvering_Unit#SAFER

I'm sorry, but astrorope made me laugh more than is probably appropriate. What a wonderfully-named item for something that I could see being pretty important. Considering the rather long and detailed names NASA gives to equipment, though, it's a great shortcut. I can imagine "Deploy your Self-Assisted Vehicular Emergency And Safety Solution (SAVEASS)!" would get a little bulky to use in an actual emergency.

If the ISS operates anything like the Shuttle, the astronauts are always tethered to something if they aren't directly holding on for dear life. You can go to NASA's website and watch spacewalk video with the radio chatter going on between the astronauts. Probably the most heard phrase in most of the videos is "attaching tether" or "detaching tether" or something similar. I'm not sure how many free walks happen on the ISS though

The jury's still out on whether there are useful amounts of water ice on the Moon

Today they're saying they found it: for example NASA's moon crash reveals water reports, "Indeed, yes, we found water," Colaprete said at a news conference Friday. "There's not just water, but lots of water."